Immunology at the Maternal-Fetal Interface

Function

Decidualization and Placentation

Decidua Formation 

The decidua is a transitory interface that is present only during pregnancy. Decidua is derived from the uterine endometrium and is located between myometrium and fetal membranes. Pre-decidualisation is noted during the luteal phase of the menstrual cycle under the influence of estrogen and progesterone. However, the implantation should occur before pre-decidualization. Therefore any disturbance in the decidualization process could cause implantation failure. 

Decidua contributes to three vital components of the maternal-fetal interface, which are listed as follows. 

1. Immune tolerance: Decidual cells are the first cells of the feto-maternal interface in contact with fetal antigens. Two distinct immune interfaces help in immune tolerance. 
• The first immune interface starts early in pregnancy, in the first trimester. It comprises the decidual immune cells, trophoblastic cells from fetal villi, decidual stromal cells, and the spiral artery.
• The second interface forms as early as eight weeks of gestation, and it comprises immune cells from the maternal circulation and syncytial trophoblast cells from the placental chorion. 
2. Trophoblast invasion: The trophoblastic cells are derived from the blastocyst part of the embryo, which lies between the embryo's external layer and the uterine layer. Trophoblasts differentiate into villous and extravillous trophoblast cells. During implantation, invading trophoblasts anchor the blastocyst to decidua, on which placentation ensues. Trophoblast invasion can also occur in organs other than the uterus, such as fallopian tubes, causing an ectopic pregnancy.[4]
3. Remodeling of spiral arterioles: After implantation, fetal trophoblast and maternal leukocytes induce the remodeling of the spiral artery, which includes the erosion of the fibrinoid wall and dilation of the vessel facilitating the placenta bathed in maternal blood. The spiral arterioles' remodeling promotes the placental function of the exchange of nutrients and waste removal.

Placenta Development

The placenta is usually formed entirely by the third week of gestation and comprises anchoring and floating villi. Placenta has the external layer of the syncytiotrophoblast and an internal layer of the cytotrophoblast. The cytotrophoblast layer forms the barrier and functions as a significant barrier between the fetal and maternal compartments. In the second trimester, cytotrophoblast thins and functionally placenta becomes hemomonochorial with only one layer of syncytiotrophoblast. 

Leukocytes At The Maternal-Fetal Interface

The maternal leukocytes at the feto-maternal interface are recruited by cytokines produced by decidual stromal cells and trophoblasts. Decidual natural killer cells (70%) and decidual macrophages(20%) account for the majority, followed by Regulatory T cells (10%).[5]

1. Decidual natural killer (NK) cells contribute to decidualization and implantation. Decidual NK cells aids in remodeling the spiral arteries and promoting trophoblast invasion via cytokines like interleukin-8. The number of decidual NK cells gradually decreases as the fetus's gestation advances and becomes less granulated.[6]
2. Decidual macrophages (DM) function as antigen-presenting cells. IL-10 and macrophage colony-stimulating factor secreted from trophoblast recruits decidual macrophage. Decidual macrophages are differentiated into pro-inflammatory M1 and anti-inflammatory M2 macrophages. The predominant phenotype at the maternal-fetal interface should be M2 macrophage, which plays a significant role in remodeling the spiral arteries and trophoblast invasion. Vascular endothelial growth factor and matrix metalloproteinase secreted from DM promote angiogenesis and tissue remodeling. DM also has an essential function of phagocytosis of apoptotic trophoblasts. DM also plays a significant role in protecting the fetus against infections.
3. Regulatory T cells (Treg) help achieve maternal tolerance to the fetal antigens and maintain a stable environment for the fetus to survive. In addition, Treg cells play a vital role in downregulating the inflammatory response for the implanting embryo. 

T-helper2 (Th2)-type Cytokines 

The maternal immune response shifts towards Th2/antibody-mediated immunity rather than cell-mediated immunity at the maternal-fetal interface. Th2-type cytokines IL-4, IL-5, and IL-10, increase at the maternal-fetal interface and make pregnancy an anti-inflammatory state embracing the semi-allogenic fetus. An infective or non-infective insult at the placental level triggering the secretion of the pro-inflammatory cytokine such as IL-6, IL-12, and TNF alpha could cause placental damage leading to preterm labor, abortion, or preeclampsia.[7]

Human Leukocyte Antigen-G

Extra-villous trophoblast expresses Human Leukocyte Antigen-G (HLA-G), which helps in the implantation of the embryo. HLA-G helps in promoting vascular permeability and angiogenesis and thereby helps in the remodeling of the spiral artery. HLA-G also stimulates NK cells to release growth factors that help in the growth of the fetus. Lower levels of HLA-G are associated with pregnancy complications such as placental abruption, pre-eclampsia, and recurrent abortions.[8]

Maternal Tolerance

Maternal tolerance plays a vital role in the successful pregnancy outcome, permitting the pregnant woman to carry her fetus with foreign fetal antigen. Failure of maternal tolerance is associated with miscarriage and preeclampsia. The modulation of leukocytes profile at feto-maternal interface helps in maternal tolerance. An excess of decidual NK cells over T-cells can be explained by epigenetic silencing of Tcell chemoattractants in stromal cells of the decidua. The cytokines like IL-10 and TGF-β promote differentiation of the existing few T cells into Treg cells in the decidua. Apoptosis is another mechanism that helps in immune privilege. IL-10 and TGF-β secreted from decidual macrophage play a major role in preventing fetal rejection and promote fetal growth till delivery.[9]

Placental Immune Defense

The placenta displays a variety of defense mechanisms to protect the fetus from most infections. They are listed as follow, 

1. The structural integrity of the placenta is the most robust defense by itself. The external layer of the syncytiotrophoblast doesn't have any cellular junctions, and also syncytiotrophoblast layer has a dense cytoskeletal network forming a dense brush border at the apical surface. So both the anatomic factor helps in defending any parasite. 
2. The antiviral cytokines secreted from the placenta also play a significant role in placental immune defense. Gamma- interferon secreted by the syncytiotrophoblast layer is a major cytokine that restricts viral infections.[10]
3. Toll-like receptors are expressed on the trophoblast, which mediates the antiinfective signaling pathway. 
4. Immunoglobulin G receptors expressed on the syncytiotrophoblast layer play a major role in the active transport of protective maternal antibodies to the fetus. This active transport begins in the second trimester and gradually increases throughout the pregnancy, with the highest peak at term.

Issues of Concern

The maternal-fetal interface is essential for the maintenance of pregnancy. It serves two primary functions listed as below,

1. It serves as an interface between the mother and fetus allowing the transfer of nutrient and oxygen; also helps in removing waste products.
2. It protects the growing fetus from the extrauterine environment, including infections.

Disturbance in the invasion of the trophoblast can cause several complications for both mother and fetus. The interruption during the establishment of the maternal-fetal interface may cause implantation failure and thereby infertility, recurrent miscarriage, or intrauterine growth retardation (IUGR).[11][12] The trophoblasts invasion is superficial in pre-eclampsia when compared to normal pregnancy.[13] In patients with unexplained infertility, endometrial biopsies have found substantially fewer NK cells than infertile counterparts. Infertility and recurrent spontaneous abortions have an indirect correlation with regulatory T cells functionality. 

The feto-maternal interface lacking gamma-interferon signaling is prone to viral infection of the fetus like the Zika virus.[14] Despite the robust placental barrier, few pathogens overcome the defense and cause severe complications to the fetus. The pathogens causing congenital infections are grouped as TORCH infections, including Toxoplasma, other [syphilis, varicella-zoster, Zika virus, human immunodeficiency virus), rubella cytomegalovirus, and herpes simplex.

Clinical Significance

Pregnancy poses a vulnerable situation for the maternal-fetal dyad putting both of them at risk of disastrous complications. Disruption of fetal tolerance may contribute to the immuno-pathophysiology for pregnancy-related maternal and fetal complications. The immune disruption at the maternal-fetal interface can explain the pathophysiology of unexplained infertility, recurrent abortion, endometriosis, preeclampsia, and placental abruption.

Recurrent Spontaneous Abortion (RSA)

RSA (three or more miscarriages) can be due to various factors, including anatomical defects and endocrine disorders ( diabetes, hypothyroidism, polycystic ovarian syndrome). Immune dysfunction at the maternal interface may also be contributing factor for recurrent abortion, as listed below.[15]

• Impaired remodeling of the spiral artery and overexpressed angiogenic growth factors are linked with RSA.
• Autophagy level is noted to be lower in trophoblastic villi among patients with RSA. 
• RSA patients have increased levels of mature dendritic cells and decreased levels of immature dendritic cells. 
• Decreased M2 macrophage and Treg cells levels are linked with RSA. 

Endometriosis

The pathophysiology of endometriosis (implantation of endometrial tissues outside the uterus) can be explained by the disturbances in the immune environment causing aberrant implantation and proliferation of endometrial tissue at the ectopic site. The following immune aberrations are noted in endometriosis. 

1. High levels of pro-inflammatory cytokines (IL-1 and TNF-α) and decreased levels of anti-inflammatory cytokines (IL-13) 
2. Increased levels of activated peritoneal macrophages
3. Lower levels of T reg cells and NK cells
4. Polyclonal B cell activation with increased antibody production.[16]

Preeclampsia

Preeclamptic pregnant women have high proinflammatory cytokines (TNF-α, IL-6, IL-8) and decreased anti-inflammatory cytokines (IL-10). High levels of CD4+ T cells infiltration in the placenta and overrepresentation of M1 macrophages are also noted among preeclamptic mothers.[17] Increased inflammatory response of the maternal-fetal interface towards semi-allogenic fetal antigens causes series of events such as impaired remodeling of a spiral artery, shallow invasion of trophoblast, and infarct of the placenta. All these factors predispose to preeclampsia.

Placental Abruption

The placental abruption is linked with sterile inflammatory changes due to insufficient decidual suppressive activity and increased levels of cytotoxic immune cells in the decidua. The pro-inflammatory cytokines and tissue-specific angiogenic factors such as vascular endothelial growth factors are upregulated, leading to disruption of vascular integrity, increasing the risk of placental abruption.[18]

Preterm Labor

Triggers such as infection, stress, or inflammation during pregnancy activate prematurely the shift of the anti-inflammatory state to a pro-inflammatory state, which precipitates preterm labor. During labor, the recruitment of effector CD4+ T cells in decidual tissues, which secretes proinflammatory cytokines such s IL-1β, and TNF-α is noted.  The number of neutrophils is increased in decidua when pregnant women have preterm labor with chorioamnionitis. The role of macrophage, NK cells, and dendritic cells in preterm labor are under investigation.[19]

Advanced Maternal Age and T cell Subset

Advanced maternal age disrupts the T-cell profile at the maternal-fetal interface and might lead to reduced fetal survival or intrauterine growth restriction. Mother of Advanced age has a lower number of CD4+ T-regulatory cells at the maternal-fetal interface and an increased number of pro-inflammatory T-cells such as gamma IFN producing CD4+ and CD+8 cells in the fetus.[20]

Virus Pathogenesis and Role of Inflammation

Viral infections put pregnant women at high risk of abortions, prematurity, low birth weight, and neurologic deficits for the fetus. The shift of the immunity in pregnancy from Th1 to Th2 suppresses CD4, CD8, and NK cells, thereby suppressing cell-mediated immunity against viral infections; this explains why few viral infections such as herpes simplex, hepatitis E and influenza have severe implications among pregnant women compared to non-pregnant.[21] 

Immune Therapy During Pregnancy

1. Intravenous immunoglobulin and allogenic lymphocyte immunotherapy increased the successful pregnancy rate among RSA patients of unknown etiology getting in-vitro fertilization.[22] In addition, they can promote successful embryo implantation by increasing the percentage of Treg cells and suppressing NK cells' activity. 
2. Cyclosporine A has been used in RSA patients because of its role in increasing Treg cells. 
3. Progesterone has been proven to decrease the risk of abortions in patients with RSA. In addition, progesterone is helpful in multiple ways, such as improving the perfusion at the maternal-fetal interface and regulating the immune response. 
4. Tregs-enhancing drugs, adoptive Treg cell target therapy, and low dose interleukin are under trial for helping RSA patients. 
5. Biologic agents such as eculizumab are used as a trial medicine to manage immune dysfunction related to preeclampsia and HELLP syndrome.[23]

Enhancing Healthcare Team Outcomes

The maternal-fetal interface ensures an adaption to the semi-allogeneic fetus and also ensures embryo development. Improving health care professionals' understanding of immunology at the maternal-fetal interface will lead to better pregnancy outcomes. In addition, a detailed understanding of the immunologic pathophysiology of maternal complications such as preeclampsia, recurrent spontaneous abortion, and endometriosis helps effectively managing pregnant women.